Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, lithium secondary batteries having high energy density and voltage are commercially available and widely used. When it is desired to use the secondary battery as a power source for mobile phones, notebook computers, and the like, there is a need for a secondary battery which is capable of stably providing a constant power output. On the other hand, when it is desired to use the secondary battery as a power source for a power tool such as electric drill or the like, there is a need for a secondary battery which is capable of instantaneously providing a high power output and capable of being stable even against external physical impact such as vibration, falling, and the like.
Further, increased environmental concern has drawn a great deal of intensive research on electric vehicles (EVs) and hybrid electric vehicles (HEVs) which are capable of replacing fossil fuel-driven vehicles such as gasoline vehicles and diesel vehicles, one of the primary causes of air pollution. Although nickel-hydrogen (Ni—H2) secondary batteries are largely employed as power sources for EVs and HEVs, numerous studies have been actively made to use lithium secondary batteries having a high-energy density and a high-discharge voltage, consequently with some commercialization potential.
Generally, the lithium secondary battery is made of a structure in which an electrode assembly, composed of a cathode, an anode and a separator interposed therebetween, is impregnated within a lithium salt-containing non-aqueous electrolyte, wherein the cathode and the anode are fabricated by applying electrode active materials to the corresponding current collectors. As the cathode active material, lithium cobalt oxides, lithium manganese oxides, lithium nickel oxides, lithium composite oxides and the like are primarily used. As the anode active material, carbon-based materials are usually used.
However, the lithium secondary battery using a carbon-based material as an anode active material suffers from deterioration of discharge capacity due to the occurrence of irreversible capacity in some of lithium ions inserted into a layered structure of the carbon-based material upon initial charging/discharging of the battery. Further, since the carbon material has a redox potential of 0.1 V which is lower relative to the Li/Li+ potential, decomposition of a non-aqueous electrolyte occurs on the anode surface, and the carbon anode reacts with the lithium metal to form a layer covering the carbon material surface (passivating layer or solid electrolyte interface (SEI) film). The SEI film may also affect charge/discharge characteristics of the battery because thickness and interface conditions of the SEI film are variable depending on the types of electrolyte systems to be employed. Further, in the secondary battery which is used in the fields requiring high output characteristics, such as power tools, the battery internal resistance increases even with such a thin SEI film, which thereby may be a rate determining step (RDS). Further, due to the formation of a lithium compound on the anode surface, the reversible capacity of lithium intercalation gradually decreases with repeated charging/discharging cycles, thus resulting in reduction of the discharge capacity and deterioration of cycle characteristics.
Meanwhile, a great deal of study has been focused on a lithium titanium oxide as a promising candidate for an anode material having structural stability and good cycle characteristics. The lithium secondary battery comprising such a lithium titanium oxide as an anode active material exhibits substantially no electrolyte decomposition due to a relatively high redox potential of the anode of about 1.5 V as compared to the Li/Li+ potential, and excellent cycle characteristics due to stability of the crystal structural. Unfortunately, the lithium titanium oxide has drawbacks such as low capacity per unit weight and low energy density.
In order to cope with such disadvantages and problems, some of conventional prior arts suggest the use of an anode material containing a carbon-based material and a lithium titanium oxide.
For example, Japanese Unexamined Patent Publication No. 1998-069922 discloses an anode with addition of a lithium titanium composite oxide as a major active material and an active material having a low redox potential as a minor active material. Further, Japanese Unexamined Patent Publication No. 2006-278282 discloses a technique with incorporation of spinel-type lithium titanate as an anode active material and a carbon material as a conductive material. However, anode materials using the lithium titanium oxide as a main active material still suffer from the problems associated with poor capacity and low energy density of the lithium titanium-based oxides.
Meanwhile, Japanese Unexamined Patent Publication No. 2001-216962 discloses a technique with incorporation of a carbon material as a major anode material and a lithium titanium composite oxide as an auxiliary active material. Further, Japanese Unexamined Patent Publication No. 2006-066298 discloses a lithium secondary battery comprising a non-aqueous electrolyte with incorporation of at least one lactone having a melting point of below 0° C., wherein an anode active material contains a carbon material capable of performing intercalation and deintercalation of lithium ions and lithium titanate, a content of the carbon material is in a range of 80 to 99% by weight based on the total weight of the anode active material, and a content of lithium titanate is in a range of 1 to 20% by weight.
However, since lithium ion mobility of the carbon material is still low due to use of the carbon material and the lithium titanium oxide in the form of a simple mixture, the above-mentioned conventional arts suffer from the problems associated with sluggish reaction rates and poor large-current characteristics.
To this end, there is an urgent need for development of an anode material which is capable of attaining low internal resistance, high electrical conductivity and excellent output characteristics while compensating for respective disadvantages of the carbon material and the lithium titanium oxide.